Electron emission system

ABSTRACT

A storage system for the mass recording and readout of digital data with ultra high resolution. An electron beam structure is provided for forming a beam of extremely small focused spot diameter, on the order of 0.1 microns, and high current density capability, on the order of 1,000 amperes per sq. cm., which records data by scanning over defined areas of the storage medium surface and micromachining elemental portions of said medium as a function of beam modulation. Readout may be subsequently accomplished by similarly scanning the beam at reduced power density and detecting electrons that have been transmitted by or reflected from the storage medium.

nited States Patent 1191 Wolfe et al. June 4, 1974 [5 ELECTRON EMISSIONSYS TEM 3,631291 12/1971 Favreau 3l3/336 s, 2 .3 s [75] inventors: JohnE. Wolfe, Cam1llus; George E. 3 678 325 7/197 N'Shldd ct W336 Ledges,Liverpool; Homer H. Glascock, Scotia, a" of Primary Exam1ner-James W.Lawrence ASSISIHHI Exammer-Saxfield Chatmon, Jr. Asslgneei GeneralElecmc p y Attorney, Agent, or Firm-Richard V.. Lang; Frank L.

Syracuse, NY Neuhauser; Oscar B. Waddell' [22] Filed: July 1, 1971 1211Appl. No.: 158,768 [57] ABSTRACT A storage system for the mass recordingand readout 62 v Related; 8 pphcjuon Data of digital data w1th ultrahlgh resolution. An electron 1 of 84797 I969 beam structure is providedfor forming a beam of extremely small focused spot diameter, on theorder of [52] U.S. Cl 3l3/336, 3i3/346, 313/174 01 microns, and highCurrent density capability, on lint. the order of amperes p q whichrecords Fleld of Search 3 data Scanning Over defined areas f the storageI f dium surface and micromachining elemental portions [56] Re erenvcesCue-d of said medium as a function of beam modulation.

UNITED STATES PATENTS Readout may be subsequently accomplished by simi-2,156, 752 5/1939 Daene .3. 313/336 7 larly scanning the beam at reducedpower density and 2.786.955 3/1957 Trolan 313/336 X detecting electronsthat have been transmitted by or 3159.782 7/1966 Shl'Off l 3l3/336reflected from the torage medium 3,356,887 l2/l967 Heil ct al. 313/336 X3,374.386 3/1968 Charbonnier et al 313/336 X 6 Claims, 11 DrawingFigures PATENTEDJun 41am SHEET 3 BF 3 LOGIC NETWORK snimumwmz,

1 ELECTRON EMISSION SYSTEM This is a division of application Ser. No.847,972, filed Aug. 6, 1969.

BACKGROUND OF THE INVENTION 1. Field of the Invention The inventionrelates to the field of mass storage and retrieval systems for storinghuge quantities of high resolution data and, in particular, to highdensity systems of this type employing electron beam recording andreadout.

2. Description of the Prior Art A figure of merit of data bits capacityhas evolved for mass storage and retrieval systems. A capacity of thismagnitude is considered desirable in order to satisfy present dayarchival storage requirements and workers in the field continue to beengaged in finding the most effective means for its accomplishmentwithin a confined space. The principal limitation of storage systemsdeveloped to date is the resolution of the storage data. Lacking veryhigh resolution, these systems require extensive storage space forstoring large quantities of data'and access times to said data arenecessarily slow.ln order to index elemental data bits for eitherstorage or retrieval purposes, a relatively complex and slow mechanicalaccessing operation is normally required. In one system known to the arta laser beam is employed for recording and reading out data on aphotosensitive storage medium which is in the form of a long continuoustape. Information is written by scanning'the beam diagonally across thetape as the tape moves longitudinally, providing parallel scan linesthroughout the tapes length. Because of inherent limitations in thewavelength of laser energy, on the order of 5,000 to 6,000 angstroms,data can be recorded with resolutions nogreater than a few microns.Thus, for a 10' bit memory, 2,400 feet of 8 mm tape must be provided. Itmay be appreciated that lengthy access times are required for thissystem.

Another existing approach is to employ an electron beam for writing onphotographic film. This has been accomplished employing a beam having a3 micron resolution and recording on 35 mm chips. For a 10 bit storagecapability there are required 200,000 individual chips. The mechanicalaccessing requirements for this system are extremely complex. Further,it is necessary to develop the photographic film before the informationcan be read out or checked, and data cannot be subsequently entered.

In the field of electron microscopy it has been suggested to employ ascanned electron beam of extremely small spot diameter, such aspresently utilized in scanning electron microscopes, to record digitalinformation by means of selective etching or a comparable technique. Theart has been developed to where there presently exist beam formingapparatus which generate beam spot diameters on the order of severalhundred angstroms and less. corresponding to a resolution orders ofmagnitude higher than in the above noted systems. However, these areallrelatively low power density beams, not capable of providing directlypermanent data storage using presently available recording materials.Accordingly, there does not exist at the present time apparatus forgenerating electron beams of minute dimensions for an ultra highresolution re-' cording which also have extremely high current densitycharacteristics for a permanent data storage.

SUMMARY OF THE INVENTION It is accordingly a principal object of thisinvention to provide a novel ultra high density storage system whichpermanently and directly records data at appreciably higher resolutionsthan presently obtainable, making possible the rapid storage of hugequantities of data within a confined space.

It is a further object of the invention to provide a novel storagesystem as above described wherein recording and readout operations areaccomplished at high speed, and do not require a separate, developingstep.

A further object of the invention is to provide a novel storage systemas described wherein exceedingly large quantities of data, on the orderof 10 bits and greater,

can be stored on a single, relatively small, storage surface.

It is still another object of the invention to provide a novel ultrahigh density storage system as above described in which data is recordedby micromachining elemental portions of the storage medium by means of ascanned electron beam, the stored data being read out also by a scannedelectron beam.

A still further object of the invention is to provide a novel electronbeam forming structure for generating a beam of extremely small spotdiameter, on the order of 0.1 microns, and high current density, on theorder of 1,000 amperes per sq. cm.

A yetfurther object of the invention is to provide a novel electron beamforming structure for generating a beam with the above notedcharacteristics by means of a relatively low accelerating voltage, onthe order of 5 to 10 KV.

It is another object of the invention to provide a novelv electronemission system having a field aided thermionic cathode that emitselectrons with an exceedingly high current density and is extremelystable in its operation.

Still another object of the inveniton is to provide a novel field aidedthermionic cathode employing a tungsten needle coated with an atomiclayer of zirconium which exhibits an extremely long lifetime, on theorder of 1,000 hours and greater.

It is yet a further object of the invention to provide a novel electronoptical system for forming a high current density, small spot diameterbeam that is capable of being deflected over a relatively large numberof resolvable elements.

Still a further object of the invention is to provide a novel electronoptical system as above described in which the effective sphericalaberration of the focus lenses-ismade low for relatively large focallengths.

In accordance with these and other objects of the invention there isprovided an ultra high density storage system which employs a scannedelectron beam of ex tremely small spot size and high current density torecord data on a storage medium by micromachining elemental portions ofsaid medium. Readout of the stored over each of which the electron beamscan be magnetically or electrically scanned. Mechanical drive means areprovided for indexing the data blocks with respect to said beam for bothwrite and readout operations.

With respect to one specific aspect of the invention, the electron beamis produced by a novel electron emission system through a process offield aided thermionic emission. The electron emission system iscomposed of a cathode including a filamentary hairpin with a weldedsingle crystal oriented tungsten needle which is of extremely smalldimension at the emissive cathode tip. Sintered zirconium is applied ina ball at the base of the needle, and upon heating of the hairpin andneedle migrates as a solid up the needle to the tip. The zirconiumcoating acts to reduce the work function of the I face on the tip of thetungsten crystal to a value appreciably lower than occurring on otherfaces. The emission system further includes an apertured anode and gridelectrode structure for generating a spherical electric fieldconfiguration about the emissive cathode tip which exhibits a very highfield gradientat said tip for causing a high power density electronemission.

With respect to a second specific aspect of the invention, an electronoptical system is provided which includes first and second focus lensesto provide a single stage imaging of the electrons emitted from thecathode tip onto the target. The cathode tip, which is at about theobject plane, is positioned at the focal point of the first lens and thetarget, which is at the image plane, is positioned at the focal point ofthe second lens. The focused beam impinges on the target at suffcientlyhigh power densities to vaporize away portions of the target material.Modulation of the beam is accomplished by a modulation coil which shiftsthe beam axis with respect to a limiting aperture provided in thevicinity of the beam source. A set of deflection coils are providedforward of the final focusing lens for deflecting the beam in both X andY directions in the plane of the storing medium.

BRIEF DESCRIPTION OF THE DRAWING The specification concludes with claimsparticularly pointing out and distinctly claiming the subject matterwhich is regarded as the invention. It is believed, however, that bothas to its organization and method of operation, together with furtherobjects and advantages thereof, the invention may be best understoodfrom the description of of the preferred embodiments, taken inconnection with the accompanying drawings in which:

FIG. 1 is a schematic diagram in a partially broken away perspectiveview of an electron beam ultra high density storage system in accordancewith one embodiment of the invention employing a limited area storagemedium;

FIG. 2 is an enlarged side view ofthe storage medium and electrondetector employed in the system of FIG. 1;

FIG. 3 is a partial plan view of the storage medium employed in thesystem of FIG. 1, illustrating the written data format;

FIG. 4 is a series of graphs illustrating the formation of the inputmodulation signal;

FIG. 5 is a side view of a modified reflective readout structure;

FIG. 6 is a detailed cross sectional view of the total electron beamstructure of FIG. 1;

FIG. 7 is an enlarged cross sectional view of the electron emissionstructure of FIG. 6;

FIG. 8 is a further enlarged cross sectional view of the cathodestructure;

FIG. 9 are several curves illustrating field aided thermionic emission;

FIG. 10 is a schematic diagram in partially broken away perspective viewof a further embodiment of the invention employing a large area storagemedium and mechanical drive means for indexing said storage medium; and

FIG. 11 is a partial plan view of the storage medium employed in thesystem of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, there isschematically illustrated in perspective view an electron beam storagesystem for permanently storing data at ultra high densities, exceeding10 data bits per sq. cm. The data is written by a scanned electron beamof extremely small beam spot diameter, on the order of O.l micron, andextremely high current density. A magnetic deflection was employed inthe illustrated system, although an electrostatic deflection might alsobe used. The writing beam, at the focusedspot, exhibits a currentdensity of IO amperes per sq. cm. and greater, and therefore a powerdensity of 10 watts per sq. cm. and greater within moderate anodevoltages. The writing beam is modulated as a function of the input dataso as to selectively micromachine by vaporization elemental portions ofthe medium 1 as the beam is scanned over its surface. Scan rates as highas l0 data bits per second and higher are employed. A non-destructivereadout of the stored data is accomplished by a scanned readout electronbeam operated at reduced power levels, about one tenth that of thewriting beam.

The electron beam structure for both record and readout includes anelectron emission system 3 and an electron optical system 4 associatedwith an evacuated chamber 5 within which the beam is enclosed. Theemission system 3 produces the electron beam and includes among itsprincipal components a cathode structure 7, grid electrode 8 and anodeelectrode 9. The electron optical system 4 controls, focuses anddeflects the beam and principally includes focusing coils 10A and 10B,deflection coils 11 and modulation coils 12. The emission system 3 andelectron optical system 4 include features of novelty that make possiblethe extremely high resolution, high current density properties of thefocused electron beam, and will be described in greater detail whenconsidering FIGS. 6 and 7. An input means 13, shown in block form,supplies input signals for modulating the beam. Input means 13 maycomprise a conventional source of digital data, e. g., the peripheralequipment of a digital computer. Although, in the disclosed embodimentsofthe invention the modulation signal contains digital data, theinvention need not be so limited and may be useful with other data formssuch as analog and alphanumeric data.

A medium vacuum, on the order of 10 mm. Hg., is provided within thechamber 5 by a vacuum pump 14, schematically illustrated in block form.A vacuum of this magnitude represents a compromise that it is relativelyeasy to achieve and maintain, while being compatible with the high orderof performance required of the present system. Also housed within thechamber 5 is the readout structure in the form of an electron detectorupon which the storage medium 1 is deposited. The storage medium 1 anddetector 15 are mounted on a base structure 16. An output means 17 isconnected to the detector 15 for receiving the readout data. A mastercontrol logic network 18 is coupled to input means 13 and output means17. The network 18 includes numerous logic circuits of standard designfor providing the sundry logic functions which control the writing andreadout operations, as well as a clock frequency generator for supplyinga master timing of said operations.

The electron detector 15 may be a conventional component. In theembodiment being considered it is a single crystal silicon p-i-njunction device which in response to penetrating electrons of thereadout beam generates electron-hole pairs. As shown in the side view ofFIG. 2, the silicon detector includes a p region 20, an intrinsic region21 and an'n region 22, with contacts 23 and 24 made to the p and nregions, respectively. The storage medium I is deposited as a thin filmover the p region 20. A dc. voltage source 25 is connected througha leadresistor 26 to contacts 23 and 24 for reverse biasing the device. Outputterminals 27 are coupled through a capacitor 28 to contacts 23 and 24for sensing readout current flow through the device 15.

During the write operation the electron beam is operated at high currentand power densities. As it scans across the surface of the storagemedium I, the beam selectively micromachines elemental portions thereof,corresponding to the data bits, as a function of beam modulation. Aheating and rapid vaporization of said elemental portions of thematerial occur in response to penetration by the beams high velocityelectrons. With respect to the vaporization process, the high densityelectrons of the focused beam spot penetrate the storage material withrelatively high kinetic energy. As the electrons are rapidly deceleratedby the bulk of the material, heat is given off which in a localized areaelevates the temperature to a point well above the threshold temperatureof the vapor pressure versus temperature curve of the material, thethreshold temperature being that temperature at which the vapor pressurecommences to rise. steeply as a function of temperature. At thiselevated temperature, the vapor pressure of said localized area israised orders of magnitude above the ambient pressure and rapidvaporization of the material results.

It is preferred to modulate the beam by applying the input signal to themodulation coils 12 for recurrently shifting the beam from its centralaxis so as to be partially intercepted during its travel, therebymaintaining a constant current emission while modulating the currentdensity at the focused spot. The beam is rapidly scanned by thedeflection coils 11 along parallel data tracks successively formed onthe surface of the storage medium.

In the embodiment of FIG. 1, the storage medium 1 has a storage areaabout 36 mils square with about 4,500 data lines and about 4,500resolvable elements per data line. Thus, there is provided a storagecapacity in excess of 2 X 10 bits. With reference to FIG. 3, a smallarea of the storage medium I in a greatly magnified plan view isillustrated, showing four data strips 31, 32, 33 and 34, the edges ofwhich comprise the data lines. Tracks 35, formed between the datastrips, are

employed to servo the readout beam as will be further explained.-Information is written along both edges of each data strip at resolvableelements on the data lines,

suchv as shown with respect to resolvable elements m and n. Inaccordance with standard practice, the data lines have a string of knowndata bits written at the beginning of each line which are used as areference during readout. lnthe present format the data strips are oncenters spaced apart by 0.4 microns, and each resolvable element is 0.2microns in length. Digital information of binary l s and 0s is writtenas a 180 phase modulation of a square wave at one half the referenceclock frequency supplied by the logic network 18, so as tocorrespondingly micromachine the data strips during either the first orsecond half of travel of the beam through each resolvable element. Thewriting beam may vaporize essentially the entire thickness of thestorage medium or only a fraction of this thickness.

. Referring to the diagram of FIG. 4, the binary bits may be fed in as aseries of l s and 0"s at two corresponding d.c. levels, as shown by theGraph A. It will be assumed that the bits are supplied at a 10 MHz rate.The clock signal, shown in Graph B, is at twice the data rate or 20 MHz.Graph C illustrates the phase modulated square wave corresponding to thedata bits in Graph A that is employed as the input signal for modulatingthe beam. With clock signal at a 20 MHz rate, a single data bit iswritten in 0.l microseconds.

For the operation under consideration, the storage medium 1 must be astable material capable of being selectively and rapidly vaporized atthe requisite resolution of the system by the writing electron beam, andyet be totally unaffected by the lower energy readout beam. Principalproperties of the medium 1 include a relatively high vapor pressure atthe writing temperature, and a vapor pressure that is a steep functionof temperature above threshold; a high density; and a low thermalconductivity. A high vapor pressure of the writing temperature causesthe material to rapidly vaporize in response to heating by the writingbeam. The steep vapor pressure versus temperature function permits areduced power readout operation that can produce no vaporization of thematerial. High density and low thermal conductivity properties permit alocalized heating of the material necessary for high resolution writing.The assignment ofspecific values for the above noted properties isdependent upon the writing and readout beam parameters as well as systemrequirements for resolution, speed, etc., and the interrelationship ofthese factors. For example, the requirements for high vapor pressure,high density and low thermal conductivity are inversely related to thewriting beam current density.

In accordance with existing performance specifications for a givingsystem, the storage medium may be selected from =various classes ofmaterials including semimetallic, semiconductor and dielectricmaterials. In the embodiment under consideration there is employed analloy of selenium with 10-20% arsenic for retaining an amorphous form ofthe selenium. This material has a specific gravity of about 4.3, athermal conductivity of about 10* calories per sec. per cmC, and

a vapor pressure of about 10 mm. Hg. at a writing temperature of 700C,which pressure reduces to about l0 mm. Hg. at a readout temperature ofC. The material is deposited on one surface of the electron detector 8as a thin film, having a thickness of approximately 1,500 to 3,000 A.

Considering the readout operation, the electron beam is operated atreduced power density. This may be accomplished by several differentmeans, but it is preferable to reduce the current density of the focusedspot through partial interception of the beam by biasing the beam offits central axis. The reduced power density causes a correspondinglyreduced heating of the storage medium, and a very greatly reduced vaporpressure, as previously noted. In the embodiment of FIG. 1, as thereadout beam is scanned the electrons from the beam are transmittedthrough the etched portions of the data elements and penetrate theelectron detector 15. Electron-hole pairs are created by the penetratingelectrons which generate a corresponding readout signal from outputmeans 17. The readout signal contains the readout data in the form ofphase information similar to the input signal. To maintain accuracy ofthe readout signal, this signal is synchronized with the clock frequencyduring the readout of each data bit.

In order that the readout beam precisely follow each data line, a servosystem is provided for sensing and correcting any tendency for beamoffset. One of several conventional servo techniques may be employed. Inthe storage system of FIG. 1, an edge servo system is used wherein asthe beam is scanned along a data line, displacement from the edge of thedata track is sensed and a correction signal generated. A servo unit,including a low pass filter and an error sensor to which the readoutsignal is coupled, may be embodied within the output means 17. As thebeam travel may tend to deviate from a scanned data line toward or awayfrom the adjoining servo track, a low frequency component is introducedinto the readout signal the magnitude of which is a function of the beamdisplacement. In response to said low frequency component, the servounit generates a correction signal which is coupled to the verticaldeflection coils for compensating the beams travel.

in FIG. is illustrated an alternate embodiment of the readout structureof the reflective type. In this embodiment, a storage medium 41 isdeposited upon a supporting substrate 42, e.g., of glass. An electrondetector 43 is positioned above the storage surface and offset from theimpinging electron beam. The electron detector 43, which may be one ofseveral conventional types including the p-i-n junction deviceillustrated in FIG. 2, receives readout electrons which are reflectedfrom the storage medium surface. The back scattered electrons can bereflected primary or secondary electrons, or both. An acceleratingpotential, not shown, is applied in known fashion to the detector forsensing secondary electrons. The readout operation is otherwise similarto that previously described, the detector responding to the reflectedelectrons for generating electron-hole pairs within its volume, which inturn provides a corresponding readout signal. Other forms of electrondetectors such as channel multipliers and photon devices may also beemployed.

A cross sectional view of the total electron beam structure includingthe electron emission system 3 and the electron optical system 4 isshown in FlG. 6, taken along the plane 66 in FIG. 1. An enlarged crosssectional view of the electron emission system per se is illustrated inFIG. 7, and a further enlarged view of the cathode structure is shown inFIG. 8. The electron LII beam structure of FIG. 6 and 7 forms anelectron beam having a theoretical current density j at the focused spoton the target that may be defined by Langmuirs equation as follows:

where j is the emission current density at the cathode emissive surface;

e is the electron charge;

V is the voltage at the target;

K is Boltzmans constant;

T is the absolute temperature; and

a is the half angle at the focused spot.

With reference to the above equation, it may be appreciated that therequirements of the storage system impose a number of significantconstraints on the electron beam structure in providing extremely hightarget current densities. Thus, a prime consideration for obtaining ahigh target current density j is to maximize the cathode emissioncurrent density j,,. The current density j is also proportional to thetarget voltage V. However, the voltage V also determines the velocity atwhich the electrons strike the target and an excessively high voltagewill result in expanding the elemental heated portions of the storagemedium and degrading resolution. Thus, the value of V must be determinedwith these conflicting considerations in mind. From the above equationit is also seen that the current density j is inversely proportional tothe temperature T, which places a limitation on heating of the cathode.

Referring to FIG. 7, the cathode structure 7 includes a hairpin filamenthaving a cathode needle 51 joined at the vertex of said hairpin. Apotentiometer, including a dc. source 52 in shunt with a resistor 53, iscoupled to the terminals of the filament 50 for heating said filament. Anegative high voltage source V is coupled to a tap on the resistor 53. Ashield 54 surrounds the cathode, grid and a part of the anode structure.The grid electrode 8 is in the form of a disk having an aperture 55through which the cathode needle extends. A negative voltage source V iscoupled to the grid 8, where V is slightly more negative than -V,. Theanode electrode 9 is of the re-entrant type, the reentrant portion beingprovided with a central aperture 56 positioned immediately forward ofthe cathode lip. The anode electrode 9 is at ground potential, as is allstructure forward of anode. At the opposite or forward end of the anodeelectrode is a limiting apertured electrode 57 in the shape of a diskhaving a central limiting aperture 58. A cylindrical sleeve 59 enclosesthe described emission structure. Several passages in the grid and anodestructure, such as at 67, 68 and 69, facilitate evacuation of theelectron emission region. The anode electrode, grid electrode andcathode needle structure together with the potentials applied theretoproduce a hemispherical electric field configuration around the cathodetip, with the tip at the radial center of the hemisphere. Thehemispherically configured electric field in combination with theextremely small dimensions of the cathode tip produce a very highelectric field gradient in the vicinity of said tip. The hemisphericalfield also limits aberrations in the focused beam.

In one operable structure, in accordance with the invention, the cathodeneedle 51 is about 30 mils in length and extends forward of the gridelectrode 8 by about 10 mils. This is the dimension g in FIG. 8. The

grid electrode shields the hairpin and assists in limiting 9 emission tothe tip of the cathode needle, well as in shaping the hemisphericalelectric field. The emissive surface at the cathode needle tip has aradius of about 1 micron. The grid electrode aperture 55 is about milsin diameter. The anode electrode 9 is about mils forward of the gridelectrode 8, shown as the dimension h in FIG. 8. The anode electrode isabout 1.125 in. wide at the forward end and has a total length in theaxial direction of about 1.1 in. The anode aperture 56 is about 10 milsin diameter and the limiting aperture 58 is about 20 mils in diameter.For the indicated length, dimension and forward extension of the cathodeneedle 51, the grid to anode spacing and the dimensions of the grid,anode and limiting apertures, the voltage V was at 5.0 KV and thevoltage -V at 5.3 KV. An electric field gradient of 10 V/cm, was therebyprovided at the cathode tip. For a voltage V of 10.0 KV and V of 10.3KV, the grid to anode spacing is modified, computed to be about mils,for retaining the 10 V/cm. electric field gradient at the cathode tip.The filament was heated to a temperature of approximately 1,800K. Thistemperature keeps the cathode tip clean of contaminating absorbed atomsin that the high field gradient of 10 V/cm. is obtained I with amoderate anode voltage of less than 5 KV to about 10 KV. These values of.voltage, particularly at the lower end, are found' not to provide anexcessively great velocity of electrons striking the present targetwhich might cause diffuse heating of the target such as to degraderesolution. In addition, for extremely thin targets, overly highvelocity electrons may penetrate completely through and not generatesufficient heat in the target material.

In accordance with the operable embodiment under consideration, thehairpin filament is composed of rhenium selected for its refractory andductile characteristics. The filament has a diameter of about 10 milsreduced to 7 mils at the vertex, as indicated in the enlarged drawingofFIG. 8. The cathode needle 51 is an covers the tip, providingcontinuous replenishment for the effects of evaporation and ionbombardment. An atomic layer of zirconium is thereby coated over thesurface of the needle 51 which, together with oxygen atoms from theresidual gas in the vacuum, act to reduce the work function at theemissive tip from 4.5 ev

for pure tungsten to 2.8 ev. The reduced cross sectional dimension ofthe filament 50 at the vertex raises the temperature of this regionrelative to the remaining length of the filament and assures migrationof the zi'rconium along the needle 51 in the direction of the tip. Theamount of zirconium material that need be dispersed is very little. Afilament temperature of 1,800K in a medium vacuum of about 10' mm. Hgkeeps the cathode tip clean of absorbed atoms. The described structureresults in cathode lifetime that is extremely long, e.g., on the orderof 1,000 hours and greater. It is noted that the optimum filamenttemperature is a function of pressure and for medium vacuum may exist ina range, typically, of l,750l( to l,850l(.

In FIG. 9 there are illustrated several field aided thermionic emissioncurves for both zirconium coated tungsten cathodes and plain tungstencathodes at different filament temperatures and for a fixed vacuum. Thecurves are plotted as emission current density in amperes per sq. cm.vs. electric field gradient .in volts per cm. Curve A represents a puretungsten cathode heated to a temperature of 2,000K. The curve is seen tocross the 10 V/cm. field gradient line at a current density of about 1 0amperes per sq. cm. Curve B represents a zirconium coated tungstencathode heated to a temperature of l,500l(, which is seen to intersectthe 10 V/cm. line at a current density of about 200 amperes per sq. cm.It is noted that although the filament temperature is lower than forcurve A, the lowered work function of the zirconium coated tungstenelement appreciably increases the current emission. Curve C represents apure tungsten cathode-heated to a temperature of 2,600K, which crossesthe 10 V/cm. line at a current density of about 500 amperes per sq. cm.It is seen that the elevated filament temperature raises the currentemission of the pure tungsten cathode above that of curves A and B.Curve D represents a pure tungsten cathode heated to a temperature of3,000K, which provides an emission current density in excess of 1,000amperes per sq. cm. at the l0 V/cm line.'Although high emissiondensities are achieved, the temperature of curves C and D are found tobe excessivelyhigh so as to drastically limit the lifetime of thecathode. Curve E represents a zirconium coated tungsten cathode heatedto a temperature of 1,800K, which is the type employed in the describedembodiment. It is seen that this cruve attains an emission currentdensity only slightly less than that of curve D but at a very much lowertemperature. Thus, at 1,800K it is found that a high emission densityand high target current density is attained, and a stable operation withlong lifetime provided.

Referring again to FIG. 6, a pair of modulation coils 12 of standarddesign are mounted on opposing surfaces of a cylindrical sleeve 59 ofthe vacuum chamber 5, the sleeve being shown also in FIG. 7. Themodulation coils are employed to direct the beam along a single axis inthe X-Y plane, which is a plane transverse to the central axis Z of thebeam. Forward of the anode electrode 9 there is mounted a first magneticfocus lens in theform of focus coil 10A which is wound about thecircumference of the chamber 5 and produces a magnetic fieldpredominantly along the central axis of the beam. The coil 10A is per seof conventional type with its conductors enclosed by a magnetic ringcoil form. A gap 60 in the inner wall of the magnetic form locates thereference plane of the focus lens, which is at the middle of the gap atplane 61. The reference plane is used for spatially relating the focuscoils one to the other and to the object and image planes. The referenceplane is used for this purpose rather than the con cept of principalplane because for these lenses the principal planes are not readilylocated. Forward of the first focus coil 10A is a second focus coil 108similar to the first, having a gap 62 in the coil form that places thereference plane of the second focus lens at plane 63. An astigmator coil64, of standard design, is wound about the chamber for generating aproper axial magnetic field in the region of the limiting aperture 58.The astigmator coil is employed to compensate any astigmatism that maybe produced by the focus coils A and 108. In addition, two pairs ofcentering coils 65, mounted on opposing surfaces of the vacuum chamberwall 66 in the vicinity of the lens plane 61 are provided for directingthe beam along two orthogonally disposed axes in the X-Y plane. Thecentering coils adjust the beam to pass through the center of the secondfocus lens. Two pair of deflection coils 11 mounted on opposing surfacesof the wall 66 forward of the second focus coil 10B deflect the beam intwo orthogonal directions in the X-Y plane.

Electron optics principles of the structure shown in FIGS. 6 and 7 willnow be discussed. Electrons emitted from the emissive surface at thecathode emission under the hemispherical electric field configurationwill generally be directed along diverging paths corre sponding to radiiof the hemispherical electric field, said paths appearing to emanatefrom a point slightly behind the cathode emissive surface which may beconsidered as a virtual image of the cathode. Only a fraction of theemitted electrons are passed by the anode aperture 56, the passedelectrons being within about a l0 solid angle of the beam central axis.Of the electrons transmitted through the anode aperture 56 only a smallfraction. within a solid angle of about l, are passed by the limitingaperture 58. The first focus coil 10A transposes the diverging beam intoa collimated beam. The second focus coil 10B transposes the parallelbeam into a converging beam which is focused on the surface of thestorage medium.

Spherical aberration of a focus lens, C,,, is the most serious form oferror existing in electron optical systems, in general, with respect toproviding a sharply focused image. C, is primarily a function of lenspower, structural dimensions of the lens and accelerating voltage.Significantly, C,- is inversely related to the lens power, or otherwiseconsidered, a direct function ofthe lens focal length. The presentconfiguration of the electron optical system very appreciably reducesspherical aberration of the system by minimizing the effective sphericalaberration C, of each lens. C, is defined as follows:

where a is the distance of the object plane or image plane to theprincipal plane of the lens. and f is the fqa t t 2 t p MW...

Through the employment ofa pair of focus lenses 10A and 108, the cathodeemissive surface, corresponding approximately to the object plane. maybe located at about the focal point of the first focus lens 10A. Thus,for each lens a fand C, E C This may be contrasted with using a singlefocus lens for focusing the cathode object at the image plane where todo so the object plane and image plane must be spaced at an appreciablygreater distance than the focal length, so that a f and C C From theabove consideration, spherical aberration of the system is reduced byincreasing the power of focus coils 10A and 108, within certain limitingfactors. With respect to coil 10A, the limiting factors are principallythe physical size and configuration of the anode structure. With respectto coil 108, the

limiting factors are the placement of the deflection coils 11 and therequirement for deflecting the beam over a wide area. Where a reflectivereadout is employed, as in the embodiment of FIG. 5, a further limitingfactor is presented in positioning of the electron detector in theregion above the storage medium.

In several exemplary operable embodiments of the electron opticsstructure, each of the focus coils 10A and 108 were identical and hadthe following specifications: the bore radius R 13/ I6 in., and theratio S/D 3/13, where S is the gap width and D the bore diameter. Thespacing of the cathode needle 51 to the plane 61, which is the dimensionk in FIG. 6, was 1.5 in., the exact dimension having been dictatedprimarily by the length of the anode electrode 9. The planes 61 and 63were spaced apart by 3.5 in., dimension 1 in FIG. 6, which is sufficientto accommodate placement of the cores but is not considered to be acritical dimension. The coil 10A was provided with ampere turns NI 455at SKV, and NI= 640 at IOKV C, 4.85.

With the storage medium 1 spaced 1 in. from the plane 63, dimension 0 inH0. 6, there were provided ampere turns N! E 570 at an acceleratingvoltage of SKV, and N1 8lO at IOKV C 1.85. A spot size of 979 A diameterwas achieved. Power density at the focused spot was measured at 6.64 X10 watts per sq. cm. at 10 KV.

With the storage medium spaced 1.5 in. from the plane 63 N! E 455 atSKV, and N1 640 at lOKV C, 4.85. A spot size of 1,058 A diameter wasachieved. Power density at the focused spot was measured at 5.69 X l0watts per sq. cm. at SKV and l.l4 X 10 watts per sq. cm. at lOKV.

It may be appreciated that the angle of convergence of the beam at thestorage medium surface is an inverse function of the spacing of themedium I and the plane 63. The beam spot size is a function of a andC',, and may be expressed as \j 11 (C or"/2) 2 where d, is the idealspot size with zero error, and a is the half angle of the convergingbeam. In determining the spacing between the medium 1 and the plane 63,conflicting constraints are present. C, decreases and 01 increases asthe spacing in reduced. The selected spacing is optimized with respectto these properties, as well as the requirement for scanning over latiyl seatsa The dimensions of the coils presented above are primarily forpurposes of example and not intended to be limiting. Other size coilsmay and have beem employed, with the electrical parameters appropriatelymodified to provide operation in accordance with the present teachings.

During a writing operation the modulation coils l2 drive the beam alonga single axis in the X-Y plane, so as to shift the central axis of thebeam between a position in the center of the imiting aperture 58 and aposition offset from the center where the focused beam is partiallyblocked by the limiting apertured electrode 57. The beam is shifted as afunction of the modulation signal. With the central axis of the beam atthe center of the limiting aperture 56 the focused beam spot is ofmaximum current density and will readily machine the storage material.At the offset position, the focused beam spot current density is reducedsufficiently so that no machining of the storage material can occur.Thus as the beam is scanned along the data lines by the deflection coils11, the current intensity at the focused spot is modulated and datathereby written.

During a readout operation the modulation coils 12 are employed tofixedly bias the beam in the offset position so that the beam ispartially blocked by the limiting apertured electrode 57 for fixedlyreducing the current density of the focused spot. The beam of thereduced current density is scanned along the data lines by thedeflection coils ll for providing readout of the stored data withouteffecting any physical change or destruction of said stored data.Alternatively, the anode voltage can be reduced during readout forreducing power density at the focused spot. I

In FIG. 10 there is illustrated in a partially broken away perspectiveview a further embodiment of a stor-' age system in accordance with theinvention employing a large area storage medium 71 composed of many datablocks 72 for providing a total storage capacity several orders ofmagnitude greater than that of embodiment of HG. 1. A single data blockcorresponds to the storage area of the medium 1 in the embodiment ofFIG. 1. In the embodiment of FIG. 9, the total'storage area of thestorage medium 71 is a plane surface about 2 l X 210 square mm.,providing about 44,000 data blocks and a total storage capacity of bits.The data blocks are arranged in column and row configuration, onlyan'exemplary number being shown in the partial plan view of HG. ll.

The electron optics corresponds to the structure of FIGS. 6 and 7 andcomparable components are similarly identified but with an added primenotation. Accordingly, the. electron emission system 3 and the electronoptical system 4' are identical to the previously considered embodiment.The input network l3',output network 17 and logic network 18' may besimilar to that of FIG. 1. Readout is preferably by means of areflective structure such as shown in FIG. 5 employing the electrondetector 43' positioned above the storage surface. However, atransmissive readout similar to FIG. ll may also be employed, requiringa suitable storage area electron detector structure for supporting thepair of motor drive means 74 and 75 located outside of the vacuum. Drivemeans 74 and 75 may include motors of conventional type which positionthe substrate 73 with an accuracy of i 1 mil. A variable reactancestepping motor is suitable. Means 74 through a linear drive shaft 76drives the substrate 73 in the X direction, and means 75 through alinear drive shaft 77 drives the substrate 73 in the Y direction. Thedrive means 74 and 75 may each include a motion translation mechanism,such as a conventional ball screw-ball nut arrangement, for convertingthe motors rotational motion to the lin ear motion of the drive shafts.A bellows such as shown at 78 provides a vacuum seal around the driveshafts 76 and 77 while accommodating their linear motion.

Accordingly, in the operation of the system of FlG. 10, the motor drivemeans 74 and 75, under control of the logic network 18, provide indexingof individual data blocks 72 with respect to the electron beam structureand the electron beam. Upon a selected data block 6 being indexed, theelectron beam may provide write and readout operations precisely asdescribed with respect to the previous embodiment of the invention.

The invention has been described with respect to a number of specificembodiments primarily for the purpose of clear and complete disclosure.it should be recognized, however, that numerous modifications may bemade to the disclosed structure by those skilled in the art which wouldnot exceed the present teaching. For example, the present electronoptical system has been employed to great advantage in combination witha novel electron emission system employing a zirconium coated orientedtungsten needle having a very low work function and-capable of operatingclean in a medium vacuum so as to provide an extremely high emissiondensity and a long lifetime. The combination of the described electronoptical system and the electron 5 emission system produces at the targeta focused beam spot of extremely small dimensions and high currentdensity. Conceivably, similar operation may be achieved by the presentelectron optical system in combination with other electron emissionsystems exhibiting similar characteristics of high emission density, stability and low long lifetime in a medium vacuum, For example, a hafniumcoated tungsten cathode or a lanthanum hexaboride cathode are believedto have the inherent properties for such operation, although to date arenot known to have been suitably developed toward this use.

Further, within the concepts of present invention, the described storagesystems may employ for storing data a material whose physical state orproperties, other than or in addition to volume, are capable of beingchanged by a high power density beam, which change of state orproperties can be detected by a readout beam. in addition, the inventionis considered to embody use of a storage material capable of selectiveerasure by an electron beam.

lt is also noted that the electron beam structure described herein mayhave useful application to other than information storage systems, forexample to micromachining and micro-etching operations in the field ofmicroelectronics.

What is claimed as new and desired to be secured by Letters Patent inthe United States is:

1. An electron emission system, comprising:

a. a hairpin filamentary heater,

b. a cathode needle of single-crystal tungsten having an emissive tipjoined to the vertex of said filament, said needle provided with asingly oriented crystal face at said tip,

. a bead of zirconium coating material for reducing the work function atsaid supply oriented crystal face applied to the foregoing filamentaryneedle cathode in the vicinity of the junction between said needle andfilament, said material migrating over the needle surface and coatingthe tip as a very thin layer in response to heating of said filament andcathode needle, and means for operating said filamentary cathode in anatmosphere containing oxygen at a medium vacuum, said oxygencontinuously combining with said migrating zirconium to facilitatecontinuous replenishment of said work function reducing coatmg.

2. An electron emission system as in claim 1 wherein said medium vacuumis on the order of 10 millimeters of mercury.

3. An electron emission system as in claim 2 in which said filament andcathode needle are heated to a temperature within the range ofapproximately 1,750K to 1,850K.

4. An electron emission system as in claim 3 in which said emissive hasa radius on the order of one micron.

5. An electron emission system as in claim 1 wherein the cross sectionaldimension of said hairpin filament in the region of said vertex isreduced so as to intensify heating of this region and assist inmigration of the a high density emission.

1. An electron emission system, comprising: a. a hairpin filamentaryheater, b. a cathode needle of single-crystal tungsten having anemissive tip joined to the vertex of said filament, said needle providedwith a singly oriented 100 crystal face at said tip, c. a bead ofzirconium coating material for reducing the work function at said supplyoriented crystal face applied to the foregoing filamentary needlecathode in the vicinity of the junction between said needle andfilament, said material migrating over the needle surface and coatingthe tip as a very thin layer in response to heating of said filament andcathode needle, and d. means for operating said filamentary cathode inan atmosphere containing oxygen at a medium vacuum, said oxygencontinuously combining with said migrating zirconium to facilitatecontinuous replenishment of said work function reducing coating.
 2. Anelectron emission system as in claim 1 wherein said medium vacuum is onthe order of 10 7 millimeters of mercury.
 3. An electron emission systemas in claim 2 in which said filament and cathode needle are heated to atemperature within the range of approximately 1,750*K to 1,850*K.
 4. Anelectron emission system as in claim 3 in which said emissive has aradius on the order of one micron.
 5. An electron emission system as inclaim 1 wherein the cross sectional dimension of said hairpin filamentin the region of said vertex is reduced so as to intensify heating ofthis region and assist in migration of the coating material over theneedle surface.
 6. An electron emissive system as in claim 1 whichfurther includes field means for providing an electric field ofgenerally hemispherical configuration about said emissive tip with asufficiently high electric field gradient in the vicinity of saidemissive tip to produce a high density emission.